Re: [PATCH v5 2/3] KVM: nVMX: Validate the IA32_BNDCFGS on nested VM-entry

[Jim Mattson <jmattson@google.com>]

KVM chooses not to support MPX in the guest unless both of these
control bits are supported by the platform.

On Fri, Nov 3, 2017 at 10:13 AM, Krish Sadhukhan
<krish.sadhukhan@oracle.com> wrote:
>
>
> On 11/02/2017 11:40 PM, Wanpeng Li wrote:
>>
>> 2017-11-03 14:31 GMT+08:00 Krish Sadhukhan <krish.sadhukhan@oracle.com>:
>>>
>>>
>>> On 11/02/2017 05:50 PM, Wanpeng Li wrote:
>>>>
>>>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>>>
>>>> According to the SDM, if the "load IA32_BNDCFGS" VM-entry controls is 1,
>>>> the
>>>> following checks are performed on the field for the IA32_BNDCFGS MSR:
>>>>    - Bits reserved in the IA32_BNDCFGS MSR must be 0.
>>>>    - The linear address in bits 63:12 must be canonical.
>>>>
>>>> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>>> Cc: Jim Mattson <jmattson@google.com>
>>>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>>>> ---
>>>> v3 -> v4:
>>>>    * simply condition
>>>>    * use && instead of nested "if"s
>>>>
>>>>    arch/x86/kvm/vmx.c | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>> index e6c8ffa..6cf3972 100644
>>>> --- a/arch/x86/kvm/vmx.c
>>>> +++ b/arch/x86/kvm/vmx.c
>>>> @@ -10805,6 +10805,11 @@ static int check_vmentry_postreqs(struct
>>>> kvm_vcpu
>>>> *vcpu, struct vmcs12 *vmcs12,
>>>>                          return 1;
>>>>          }
>>>>    +     if (kvm_mpx_supported() &&
>>>> +               (is_noncanonical_address(vmcs12->guest_bndcfgs &
>>>> PAGE_MASK, vcpu) ||
>>>> +               (vmcs12->guest_bndcfgs & MSR_IA32_BNDCFGS_RSVD)))
>>>> +                       return 1;
>>>> +
>>>>          return 0;
>>>>    }
>>>>
>>> Hi Wanpeng,
>>>    The SDM check is performed only when "load IA32_BNDCFGS" VM-entry
>>> control
>>> is 1. But vmx_mpx_supported() returns true when both "load IA32_BNDCFGS"
>>> and
>>> "store IA32_BNDCFGS" VM-entry controls are 1. Therefore your check is
>>> performed when both controls are 1. Did I miss something here ?
>>
>> https://lkml.org/lkml/2017/11/2/748 Paolo hopes the simplification.
>>
>> Regards,
>> Wanpeng Li
>
> I got that simplification and your changes look good to me.
>
>
> However, I am still curious know the reason why vmx_mpx_supported() returns
> true only when both controls are true whereas the SDM states the following:
>
>    "IA32_BNDCFGS (64 bits). This field is supported only on processors that
> support either the 1-setting of the  “load IA32_BNDCFGS” VM-entry control or
> that of the “clear IA32_BNDCFGS” VM-exit control."
>
> Thanks,
> Krish